linux-kernel - Re: [3.1 patch] x86: default to vsyscall=native

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Message-ID: <CAFLxGvzpvJ1mgdcbrq+3h5EA+VOhZDR9BY27uWGgsmV4TWYb4A@mail.gmail.com>
Date:	Thu, 6 Oct 2011 00:22:34 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Adrian Bunk <bunk@...sta.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [3.1 patch] x86: default to vsyscall=native

On Thu, Oct 6, 2011 at 12:13 AM, Andrew Lutomirski <luto@....edu> wrote:
> On Mon, Oct 3, 2011 at 10:33 AM, Adrian Bunk <bunk@...sta.de> wrote:
>> On Mon, Oct 03, 2011 at 06:04:53AM -0700, Andrew Lutomirski wrote:
>>> On Mon, Oct 3, 2011 at 2:08 AM, Adrian Bunk <bunk@...sta.de> wrote:
>>> > After upgrading a kernel the existing userspace should just work
>>> > (assuming it did work before ;-) ), but when I upgraded my kernel
>>> > from 3.0.4 to 3.1.0-rc8 a UML instance didn't come up properly.
>>> >
>>> > dmesg said:
>>> >  linux-2.6.30.1[3800] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb9c498 ax:ffffffffff600000 si:0 di:606790
>>> >  linux-2.6.30.1[3856] vsyscall fault (exploit attempt?) ip:ffffffffff600000 cs:33 sp:7fbfb13168 ax:ffffffffff600000 si:0 di:606790
>>> >
>>> > Looking throught the changelog I ended up at commit 3ae36655
>>> > ("x86-64: Rework vsyscall emulation and add vsyscall= parameter").
>>> >
>>> > Linus suggested in https://lkml.org/lkml/2011/8/9/376 to default to
>>> > vsyscall=native.
>>> >
>>> > That sounds reasonable to me, and fixes the problem for me.
>>>
>>> At this point in the -rc cycle, this sounds fine.
>>>
>>> That being said, I'd like to fix it for real for 3.2.  This particular
>>> failure is suspicious -- the "vsyscall fault" message means that
>>> sys_gettimeofday returned EFAULT, which means that the old (3.0 and
>>> before) vgettimeofday should *also* have segfaulted.
>>
>> This 2.6.30.1 UML kernel binary from 2009 worked for me for all host
>> kernels from 2.6.30 to 3.0, and with 3.1.0-rc8 and vsyscall=native
>> it also seems to run nicely.
>>
>> Looking deeper into "a UML instance didn't come up properly",
>> the problem is that it comes up in a strange (readonly) state.
>>
>> There are "Using makefile-style concurrent boot in runlevel S."
>> and "Using makefile-style concurrent boot in runlevel 2." in the
>> logs with a Debian userspace, but no output from the init scripts
>> in these broken bootups (normal messages are in non-broken bootups).
>>
>> Perhaps the two the messages I see in dmesg on the host are from the
>> processes running rcS and rc2 failing early?
>>
>> In a working startup with a Debian userspace, I'm getting during rcS
>>  Setting the system clock.
>>  Cannot access the Hardware Clock via any known method.
>>  Use the --debug option to see the details of our search for an access method.
>>  Unable to set System Clock to: Mon Oct 3 17:01:35 UTC 2011 ... (warning).
>>
>>> We do have a bit
>>> of a bug in that the new code doesn't report si_addr properly, but
>>> that sounds unlikely as a culprit.  Did you try with the offending
>>> commit reverted (i.e. fce8dc0)?  I bet that it also fails there.
>>
>> fce8dc0 is "x86-64: Wire up getcpu syscall", is that really the one you
>> want me to revert?
>>
>>> What's the .config for your UML binary?  I'd like to see if I can
>>> reproduce this.
>>
>> It's attached.
>>
>
> I can't reproduce it.  What distro is running inside the UML instance?

Same here.
Adrian, is the UML kernel crashing before executing init?
We definitely need more information...

-- 
Thanks,
//richard
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